For several decades, magnetic recording devices used for recording data, such as magnetic drum memories or magnetographic printers, have included a magnetic recording carrier, most often comprising a magnetic drum or endless belt coated with a magnetic film. Recording of the data on this carrier is performed by means of a recording device that includes a plurality of magnetic recording heads, in proximity with which the recording carrier is displaced. Each time one of the heads is excited for a brief instant by an electric current of suitable intensity, it generates a magnetic field, the effect of which is to create magnetized domains of small dimensions on the surface of the recording carrier moving past the heads. These virtually point-shaped domains are typically known as magnetized points.
These heads, each of which comprises a magnetic core about which an excitation winding has been wound, are typically disposed side by side and are aligned in a direction perpendicular to the direction of displacement of the recording carrier. The excitation windings of these heads are controlled by an electrical excitation circuit, which from the utilization standpoint is most often a structure of the matrix type. In other words, in this structure, the excitation windings of the heads are distributed into p groups, each including n windings, and one of the ends of each winding is connected to one of the terminals of a source of electrical current via one of n first switch devices, each of these n switch devices being associated respectively with a corresponding winding of the n windings of each group. The other end of each winding is connected to the other terminal of the source, via one of p second switch devices, and these p switch devices are each associated respectively with a corresponding group of the p winding groups. Under these conditions, it will be understood that the electrical excitation of a winding is obtained by simultanously closing the first switch device, which is associated with that winding, and the second switch device, which is associated with the group to which this winding belongs. By definition, during normal function, as long as the excitation circuit is not detective in any way, the simultaneous closure of a first switch device and a second switch device always has the effect of causing an electrical current to flow in a circuit portion comprising these two switch devices and through the winding whose ends are connected to these two switch devices.
However, a cutoff or current interruption may occur in this circuit portion, which could be due either to a defective closure of one or more of the two switch devices of this portion, for instance from wear, or to a break in the winding or breakage of the solder connections connecting this winding to the two switch devices. In that case, if it is desired to close these two switch devices, excitation of the winding interposed between these two devices cannot be attained. Conversely, it may happen that a switch device that is placed in the closing position remains locked in that position at the end of the normal period for closure. In that case, the winding of which one end is connected to that switch device will be involuntarily excited when the other switch device that is connected to the other end of this winding is closed. This involuntary excitation translates into the recording of undesired data on the surface of the recording carrier.
To eliminate the risk of recording error following a cutoff, a bad contact, or a short circuit occurring in a portion of the electrical excitation circuit, the prior art has proposed various detection devices capable of detecting defects in the functioning of such a circuit. For instance, a known verification device described in French Pat. No. 1.496.316, in order to detect that the excitation winding wound about the core of a magnetic recording head has indeed been excited by recording control signals furnished by a control circuit, includes a supplementary winding wound about the core of the head and a comparator, the task of which is on the one hand to compare the signals that are induced in this supplementary winding with those that have been furnished by the control circuit and on the other hand to generate an error indication signal when agreement is not found between the compared signals.
Another device for detecting defective functioning is described in IBM Technical Disclosure Bulletin, Vol. 27, No. 9, February 1985, pp. 5464-5465. This device, comprising an analog circuit connected in parallel with the terminals of a resistor inserted into the circuit portion to be tested, furnishes an error indication signal when the intensity of the current flowing in this portion is above a predetermined threshold value.
Although these detection devices are suitable for detecting functional defects occurring in a circuit that includes a small number of circuit portions, still they would not be usable for detecting defects occurring in a great number of circuit portions, because the number of devices necessary for this detection would have equal to the number of circuit portions to be tested, which naturally would greatly increase the cost price of the machines that would be equipped with the device.
Furthermore, with these devices, verification of the status of a circuit portion cannot be done except if this circuit portion has an electrical current flowing through it at a intensity that equals a fixed predetermined value. Hence the status of the excitation winding of a magnetic recording head, for instance, is verified only if this winding has a current passing through it of sufficient intensity to cause the recording of a datum onto the recording carrier. If the excitation winding were cut, the absence of a signal induced in this supplementary winding of the head would indicate the presence of a defect in the excitation winding. If the current intensity flowing in the excitation winding of the head, with the winding intact, were considerably reduced, in order merely to verify the status of the winding without recording data on the recording carrier, then that device could not determine whether the excitation winding of the head is functioning properly, because the amplitude of the signal induced in the supplementary winding of the head would be zero, or at least much too feeble to conclude with certainty that the excitation winding is free of defects.
By definition, these detection devices prove to be incapable of verifying the status of circuit portions at any time other than the normal excitation periods of these portions.